First-principles calculations of thermodynamic properties of TiB2 at high pressure

The equations of state (EOS) and other thermodynamic properties of TiB₂ are investigated using ab initio plane-wave pseudopotential density functional theory method. The obtained results are in good agreement with the experimental measured data and other theoretical calculated ones. Through the quasi-harmonic Debye model, in which the phononic effects are considered, the dependences of relative volume V/V₀ on pressure P, cell volume V on temperature T, and Debye temperature Θ and specific heat C_V on pressure P are successfully obtained.     

B→ηlν decays and the flavor-singlet form factors

We study semileptonic decays B→η^(′)lν, taking into account the flavor-singlet contribution (F^singlet₊) to the B→η^(′) form factors, which arises from the two-gluon emission in a decaying B meson. It has been recently pointed out that, in addition to large weak annihilation effects, the unknown value of F^singlet₊ prevents accurate theoretical estimates in the analysis of B→η′K decays in QCD factorization. We present a certain method to determine F^singlet₊ with a reasonable accuracy, using B→η^(′)lν and B→πlν decays. We also investigate the possible effect of F^singlet₊ on the estimated branching ratios (BRs) for B→η^(′)lν and find that the BR for B→η′lν is particularly sensitive to the effect of F^singlet₊.     

On the pp → ppη(η′) reactions near threshold

The production rate for η′ in pp → ppη′ at rest is calculated in a covariant one boson exchange model, previously applied to study π⁰ and η production in NN collisions. The transition amplitudes for the elementary BN → η′N processes with B being the meson exchanged (B = π, σ, η, , ω and a₀) are taken to be the sum of s- and u-channels with a nucleon in the intermediate states, and an a₀ meson pole in a t-channel. The couplings of the η′ to hadrons are a factor 0.4 weaker than the respective η-hadron couplings, as suggested by a quark model and a singlet-octet mixing angle θ = −23°. The model reproduces near threshold cross sections for the quasielastic processes π⁻p → nη(η′) and pp → ppη(η′) reactions.     

Pressure–temperature studies on conductivity of TlX(X=Cl,Br,I) compounds:: I: phase diagram. II: ionic mobility

We have measured the conductivity σ of TlX(X=Cl, Br, I) compounds up to 5.3 GPa and between 300–823 K. The σ–T dependence for all compounds can be divided into three distinct regions: (i) low temperature (LT), <400 K, with unusual negative σ–T dependence, (ii) intermediate temperature (IT), 400<650 K, with positive σ–T dependence and (iii) high temperature (HT), T>650 K, with positive σ–T dependence. The σ–T isobars were used to construct the T–P solid phase diagram for each compound. The LT region data indicate a new meta-stable phase in the 1.0–3.5 GPa range. The LT→IT transition is characterized by an inverse σ–T dependence followed by normal Arrhenius behavior up to and including the HT region. The extrapolation to 1 atm of the P-dependent boundary between IT and HT regions above 3 GPa for each compound in the P–T plot yields a value close to its respective normal (1 atm) T_melt suggesting a solid order–disorder transition type paralleling -AgI behavior. The abrupt drop in conductivity in the LT region for P between 2.5–4.1 GPa of all compounds is at variance with the Arrhenius behavior observed for unperturbed ion migration implying the appearance of a second factor overriding the Arrhenius temperature dependence. Normal Arrhenius σ–T dependence prevails in both IT and HT regions with Q_c values of 85–100 kJ mol⁻¹ and 50–75 kJ mol⁻¹, respectively. The higher conductivities at 0.4 GPa for TlBr and TlI relative to their 1 atm data and the increasing σ with P are in strong contrast to the normal σ-P behavior of TlCl. The dependence of activation volume ΔV^‡ on T for TlCl, i.e. ΔV^‡>0, shows abnormally high values with a maximum at 500 K for P<3.0 GPa but reasonable ΔV^‡ values appear above 3.0 GPa. The ΔV^‡–T dependence for both TlBr and TlI with ΔV^‡<0 is incompatible with an ion transport mechanism suggesting an electronic conduction process and implying an ionic–metallic transition at higher pressures. These contrasting conductivity features are discussed and interpreted in terms of electronegativity differences and bonding character rather than structure.     

Thermodynamic properties of MgO under high pressure from first-principles calculations

The equations of state (EOS) and other thermodynamic properties of the rocksalt (RS) structure MgO are investigated by ab initio plane-wave pseudopotential density functional theory method. The obtained results are consistent with the experimental data and those calculated by others. Through the quasi-harmonic Debye model, in which the phononic effects are considered, the dependences of relative volume V/V₀ on pressure P, cell volume V on temperature T, and Debye temperature Θ and specific heat C_V on pressure P are successfully obtained. The variation of the thermal expansion with temperature and pressure is investigated, which shows the temperature has hardly any effect on the thermal expansion at higher pressure.     

Origin of the metal-insulator transition in the superconducting series Bi2Sr2Ca1−xYxCu2O8+δ

The metal-insulator transition in the solid solution Bi₂Sr₂Ca_1−xY_xCu₂O_8+δ (0≤x≤1) has been investigated by TGA (oxygen content) and by X-ray absorption spectroscopy (Bi and Cu valence states). Resistivity and AC magnetic susceptibility measurements have shown that the superconducting properties and the metallic behavior vanish for x>0.55. The oxygen content δ is larger than x/2 for x≤0.3 and smaller than x/2 for x≥0.6. For x=0, the Cu K edge shows a shift towards high energy with respect to the Cu(II) oxide La₂CuO₄; this shift decreases with increasing x in agreement with the decrease of the doping hole density and the variations of the physical properties. For 0≤x≤0.3, the Bi L₃ edge shows a shift of 1 eV towards low energy with respect to the Bi(III) oxide Bi₂O₃ in agreement with the charge transfer between [CuO₂]_∞ and [BiO]_∞ planes. This shift also decreases with increasing x, but is still present for the x=0.6 composition for which δ is smaller than x/2. A model of the metal-insulator transition in this series is proposed based on the fact that the intercalation of excess oxygen raises the bottom of the Bi-O band with respect to the Fermi level and decreases the contribution of the Bi-O electron pocket to the hole density.     

Multiplicity and average PT of central nucleus-nucleus collisions

We evaluated the multiplicity and average P_T of central nucleus-nucleus collisions using an extension of the Glauber model and theoretical ideas coming from particle-nucleus interactions. The multiplicity rises with the number of nucleons of the projectile A as c₁A+c₂A^1/3. The average P_T decreases with the number of nucleons B of the target. We discuss our results in connection with the possible phase transition between hadron matter and quark-gluon matter.     

Magnetism and hyperfine interactions of Fe, Eu, Gd, Dy, Er and Yb in RM4Al8 compounds (R = rare earth or Y, M = Cr, Mn, Fe, Cu)

Mössbauer and magnetic susceptibility studies of sixty tetragonal RM₄Al₈ compounds (R = 4f, M = 3d element), show a wide variety of magnetic phenomena in the behaviour of 3d transition elements. The rare earths order antiferromagnetically at temperatures below 10–30 K in all compounds. The 3d elements, however, all behave differently. Fe in RFe₄Al₈ has a localized moment (effective moment of 4.4 _μB) and orders independently of the rare earth sublattice. Mn in RMn₄Al₈ has also a localized moment (1 _μB) but orders only when the rare earths order. Cr in RCr₄Al₈ has no moment of its own, but it has an induced moment (.1 _μB) by its magnetic rare earth neighbours. Cu in RCu₄Al₈ is nonmagnetic. The Mössbauer studies of ¹⁵¹Eu, ¹⁵⁵Gd, ¹⁶¹Dy, ¹⁶⁶Er, ¹⁷⁰Yb and a ⁵⁷Fe probe yield all hyperfine interaction parameters including the orientation of the hyperfine field relative to the crystallographic c-axis. In addition, the studies yield the Ce, Eu and Yb valencies in the various compounds. Eu in EuFe₄Al₈ and in EuMn₄Al₈ and Yb in YbCr₄Al₈ are in a mixed valent state.     

Double-step behavior of critical current vs. magnetic field in Y-, Bi- and Tl-based bulk high-Tc superconductors

A double step characteristic is observed at 76 K in the transport critical current as a function of magnetic field (10^-4 T to 10 T) in bulk sintered Y-, Bi- and Tl-based high-T_c superconducting materials. The low-field, step-like drop in the critical current density J_c commences at magnetic fields B between about 0.3 and 2 mT. This is followed by a plateau region of relatively constant critical current extending from about 30 to 300 mT, and then a second drop at fields between about 0.3 and 10 T. These features occur for all three superconductor materials and are interpreted respectively as a self-field/weak-link regime, a remnant percolation path regime and a flux-flow/upper-critical-field regime. The sharpness of the transition of the voltage-current (V-I characteristic, represented by the transition parameter n (i.e., V∝Iⁿ), has a similar double-step shape as a function of magnetic field directly corresponding to the features of the J_c(B) characteristic.     

Proton polarization observables for the elastic and inelastic scattering of 500 MeV protons from Ca and Pb

Measurements for the elastic and inelastic scattering of 500 MeV protons from ⁴⁰Ca and ²⁰⁸Pb at small momentum transfers are reported. The induced proton polarization P and the spin rotation parameter Q were measured for the elastic scattering. The spin rotation parameters D_SS′, D_SL′, D_LL′, d_LS′ and the induced polarization P were measured for transitions to the 3⁻(3.37 MeV) and 5⁻(4.48 MeV) states in ⁴⁰Ca and the 3⁻(2.61 MeV) state in ²⁰⁸Pb. Comparisons of the data with the theoretical calculations were carried out in the framework of nonrelativistic and relativistic approaches. We extracted the spin-up and spin-down channels for the elastic scattering cross sections and found that predictions of the relativistic and nonrelativistic approaches agree well with the spin-up channel data and differ considerably in the spin-down channel.     

Matter–antimatter asymmetry and neutrino properties

The cosmological baryon asymmetry can be explained as remnant of heavy Majorana neutrino decays in the early universe. We study this mechanism for two models of neutrino masses with a large ν_μ−ν_τ mixing angle which are based on the symmetries SU(5)×U(1)_F and SU(3)_c×SU(3)_L×SU(3)_R×U(1)_F, respectively. In both cases B−L is broken at the unification scale Λ_GUT. The models make different predictions for the baryogenesis temperature and the gravitino abundance.     

Gas Cotton-Mouton constant measurement by extrapolation in dissolved binary liquid solutions

The magneto-optic Cotton-Mouton effect constant C, light refraction index n and density ρ of binary solutions of toluene in carbon tetrachloride, -picoline and β-picoline in 1,4-dioxane have been measured at different concentrations. The results have been used for the calculation of the molar C^M constants of the solutions. By extrapolating the values C^M = C^M(f₂) for the concentration f₂ → 0, the constant _∞C₂^M = _gasC₂^M of the dipolar component of the solution has been found, which is interpreted as the Cotton-Mouton gas constant. For all solutions, the reduction factors of the dissolved component have been calculated and their linear dependence on the solution concentration has been found.     

Pinning model derived from resistive measurements on melt processed Bi2Sr2CaCu2O8

On melt processed samples of the 86 K superconductor Bi₂Sr₂CaCu₂O₈ we have performed resistive measurements in the low field limit B0.13 T and 40 KT77 K. The voltage drop is found to rise exponentially with current E ∝exp j/j₀, which is interpreted in terms of thermal activation of pinned flux lines. An activation energy U₀(T)90 meV is derived from the transition width j₀(T) and is related to a plausible core pinning interaction of flux lines with normal conducting precipitates. This reproduces the measured j_c(B, T) values in the whole regime investigated. We conclude that pinning centers must have a minimum size in order to control flux creep. Finally we demonstrate that conventional summation of the single site pinning forces cannot account for the observed macroscopic depinning current density.     

Electronegativity explanation on the efficiency-enhancing mechanism of the hybrid inorganic–organic perovskite ABX_3 from first-principles study

Organic–inorganic hybrid perovskites play an important role in improving the efficiency of solid-state dye-sensitized solar cells. In this paper, we systematically explore the efficiency-enhancing mechanism of ABX_3(A = CH_3NH_3; B = Sn,Pb; X = Cl, Br, I) and provide the best absorber among ABX_3 when the organic framework A is CH_3NH_3 by first-principles calculations. The results reveal that the valence band maximum(VBM) of the ABX_3 is mainly composed of anion X p states and that conduction band minimum(CBM) of the ABX_3 is primarily composed of cation B p states. The bandgap of the ABX_3 decreases and the absorptive capacities of different wavelengths of light expand when reducing the size of the organic framework A, changing the B atom from Pb to Sn, and changing the X atom from Cl to Br to I. Finally, based on our calculations, it is discovered that CH_3NH_3 Sn I_3has the best optical properties and its light-adsorption range is the widest among all the ABX_3 compounds when A is CH_3NH_3. All these results indicate that the electronegativity difference between X and B plays a fundamental role in changing the energy gap and optical properties among ABX_3 compounds when A remains the same and that CH_3NH_3 Sn I_3 is a promising perovskite absorber in the high efficiency solar batteries among all the CH_3NH_3BX_3 compounds.     

In-band M1 and E2 transition rates and collective structures in Ba

Subpicosecond mean lifetimes of eight excited states in ¹²⁸Ba populated via the ⁹⁶Zr(³⁶S,4n) reaction were measured by the Doppler-shift attenuation (DSA) technique using a line-shape analysis. The differential decay-curve method (DDCM) was applied for the lifetime determination. The B(E2) values in the yrast band indicate that the first band-crossing is with a proton S-band. The configuration πh_11/2d_5/2 of the negative-parity semi-decoupled bands is confirmed by the measured B(E2, I → I − 2) and B(M1, I → I − 1) transition strengths. The higher-lying “dipole” band in ¹²⁸Ba can be described as a high-K four-quasiparticle band built on the prolate configuration (πh_11/2d_5/2) (νh_11/2g_7/2).     

Positivity of energy in five dimensional classical unified field theories, II

Five dimensional classical unified field theories as well as Yang-Mills theory with gauge group U(1), are described in terms of a Lorentzian five dimensional space V₅ with metric tensor γβ which admits a space-like Killing vector ζ. It is assumed that: (1) V₅ has the topology of V₄ x S¹, S¹ is a circle and V₄ is a four dimensional Lorentzian space that is asymptotically flat and (2) the Einstein tensor Γ_β of V₅ satisfies Γ_β Uυ^{β 0} where U and υ are future oriented time-like vectors with γ_βυζ^β = 0. The spinor approach of Witten [4], Nester [3] and Moreschi and Sparling [5] is used to show that the conserved five dimensional energymomentum vector P = ifΓ_β = 0 then V₅ must admit a time-like Killing vector. Lichnerowicz's results [1] then imply that V₅ must be flat. A lower bound for P⁴ (the mass) similar to that found by Gibbons and Hull [6] is obtained.     

水泥老化过程中水动态的准弹性中子散射(QENS)谱分析

准弹性中子散射(quasi-elastic neutron scattering, QENS)实验是研究水泥老化过程中水动态的一种新颖的实验方法.本文利用老化时间分别为7, 14和30 d水泥样品的QENS谱实验数据, 通过应用四个高斯项的和的能量分辨函数R(Q, E)代替一个高斯项的能量分辨函数来改进经验扩散模型(empirical diffusion model, EDM), 再进行非线性最小二乘拟合.由此导出水泥样品中水动态的相关物理参数: 不动水数密度A, 自由水指数FWI=B₁/(A+B₁+B₂), 洛伦兹函数的半高宽Γ, 移动水跳跃之间的平均停留时间τ ₀及自扩散系数D_t, 而且可得出更为精准的QENS谱拟合曲线.拟合得到的物理参数可定量描述水泥老化过程中水动态过程, 从而为QENS实验在水泥老化过程中水动态研究的应用提供一种合理实用的谱分析方法.     

On matching conditions in the WKB method

The modified matching conditions for quasiclassical wave functions on both sides of a turning point for the radial Schrödinger equation have been obtained. They differ significantly from the usual Kramers condition which holds for the one-dimensional case. Namely, the ratio C₂/C₁ in the subbarrier and the classical allowed regions is not a universal constant ( , as usual), but depends on the values of the orbital angular momentum l, energy E and on the behaviour of the potential V(r) at r → 0. The comparison with exact and numerical solutions of the Schrödinger equation shows that the modified matching conditions not only make the quasiclassical approximation in the subbarrier region asymptotically exact within the n → ∞ limit, but also considerably enhances its accuracy even in the case of small quantum numbers, n 1. The power-law, funnel and short-range potentials are considered in detail.     

The angular momentum and parity projected multidimensionally constrained relativistic Hartree–Bogoliubov model

Nuclear deformations are fundamentally important in nuclear physics.We recently developed a multidimensionally constrained relativistic Hartree-Bogoliubov(MDCRHB)model,in which all multipole deformations respecting the V₄ symmetry can be considered self-consistently.In this work we extend this model by incorporating the angular momentum projection and parity projection to restore the rotational and parity symmetries broken in the mean-field level.This projected MDCRHB(p-MDCRHB)model enables us to connect certain nuclear spectra to exotic intrinsic shapes such as triangles or tetrahedrons.We present the details of the method and an exemplary calculation for ¹²C.We develop a triangular moment constraint to generate the triangular configurations consisting of three α clusters arranged as an equilateral triangle.The resulting ¹²C spectra are consistent with that from a triangular rigid rotor for large separations between the α clusters.We also calculate the B(E2)and B(E3)values for low-lying states and find good agreement with the experiments.